While installing WordPress plugin or upgrading WordPress from your WordPress admin area, you may receive an error related to SSL certificate.
You may receive following error or similar error message in your WordPress admin area:
Download failed.: SSL certificate problem: unable to get local issuer certificate
There could be many reasons for such error but the following solution worked for me and it should help other users as well.
 Download the cert.pem file from the following URL:
Right click on “cacert.pem” as shown in following image and click on “Save Link As…”
 Move cert.pem file in your php installation folder. For example, if your PHP installation path is “C:\Program Files (x86)\PHP\v5.5\”, move cert.pem file to this directory.
 Now, open your php.ini file and search for [curl] section in your php.ini file. Add following line under curl section as follow:
curl.cainfo = "C:\Program Files (x86)\PHP\v5.5\cacert.pem"
If there is no [curl] section, you can add it and then add the above line as shown in the following image.
 Save the php.ini file and restart your web server.
Now you can attempt to upgrade your WordPress or install plugin from your WordPress admin area. This had fixed my issue and probably it should help you to fix your issue as well.